The present invention relates to a thermal head driving method for performing thermal recording in which heat generating elements juxtaposed in the thermal head along a scanning line are divisionally driven a number of times to complete one line recording.
In a thermal recorder which performs thermal recording upon heat sensitive paper or a heat-transferable ink donor sheet, a thermal head is extensively employed as a means for generating heat pulses corresponding to recording data.
FIG. 1 is an explanatory diagram for describing a conventional thermal head driving method. In the thermal head of the illustrated thermal recorder, lead wires 2 and 3 are alternately connected from opposing sides to a heat generating unit 1 at given itervals. The other ends of the lead wires 2 are connected to parallel signal output terminals of a shift register driver 4. The other ends of the lead wires 3 are connected alternately to first and second common electrodes 6 and 7 through respective diodes 5. The voltage of a power source 12 is applied to the first common electrode 6 through a first switch circuit 11 which is actuated in response to a first common drive signal 9 produced from a sequence controller 8. Likewise, the voltage of the power source 12 is applied to the second common electrode 7 through a second switch circuit 14 which is actuated in response to a second common drive signal 13 produced from the sequence controller 8.
First and second halves of one line data 15.sub.1 and 15.sub.2 are supplied as data to the sequence controller 8 at different times.
When the first half of the line data 15.sub.1 is produced from the sequence controller 8 (FIG. 2a) and set into the shift register driver 4, the first common drive signal 9 is produced from the sequence controller 8 and applied to the first switch circuit 11, for example, during 1 msec (FIG. 2b), running from the time instant when the first half of the line data 15.sub.1 has been set into the shift register driver 4. During this time period, i.e. 1 msec, half of the number of heat generating elements are driven two by two (plus one on either end) and thermal recording for half of one line is thus carried out. Thereafter, the second half of the line data 15.sub.2 is set into the shift register driver 4. Upon the setting of the second half of the line data 15.sub.2 being completed, the second common drive signal 13 is produced from the sequence controller 8 (FIG. 2c) and applied to the second switch 14, whereby the voltage is applied to the second common electrode 7 and thus the remaining half of the heat generating elements are driven two by two during the period when the second common drive signal 13 is applied to the second switch circuit 14. In this manner, the thermal recording for one line is carried out.
In the thermal recorder of the type described, if the recording paper advances in the sub-scanning direction during the operation of one line recording carried out divisionally over two periods, dots 16 recorded by the application of the voltages to the first common electrode 6 and dots 17 recorded by the application of the voltage to the second common electrode 7 are dislocated in the sub-scanning direction as shown in FIG. 3. Unless the recording positions of picture elements in one line are aligned, reproduceability of a character or a pattern made up of closely spaced segments in the sub-scanning direction, e.g. a character "E", will be extremely degraded.
Heretofore, in order to overcome this drawback, the advancement of the recording paper was halted while the thermal head was effectuating divisional driving, and a time T.sub.1 (FIG. 2a) for moving the recording paper was disposed after completion of the one line recording. Therefore, according to the conventional thermal head driving method, the time A.sub.1 (FIG. 2c) necessary for one line recording is the sum of the time for driving the heat generating elements and the time for moving the recording paper. As such, high speed recordation cannot be accomplished.